1. Field of the Invention
The present invention relates to closures for pressure vessels and, more particularly, to means for sealing closure members to access openings of high temperature, high pressure heat exchangers.
2. Description of the Prior Art
Utility power plants have numerous pressure vessels, such as feedwater heaters, which are designed for internal pressures as high as 5,000 psi, or even more, and at corresponding internal temperatures well above 400.degree. F. (or 200.degree. C.). A typical feedwater heater has a bundle of tubes arranged in a cylindrical shell, the ends of the tubes penetrating a tube sheet near one or both ends of the shell to open into a header space. Since access to the header space is necessary for periodic inspections and for replacement or repair of leaky tubes, the end of the header is usually closed by a removable cover assembly. The assembly may include a relatively thin plate or diaphragm backed by a thick cover. The diaphragm serves as a closure member or sealing member, and the cover supports the diaphragm against the internal operating pressure in the header space.
The conflicting needs to provide effective sealing over periods of many months and yet to obtain ready access for inspection and maintenance have never been satisfactorily resolved. In some units flat gaskets are clamped between the rim of the diaphragm and an annular shoulder formed in the access opening. This arrangement requires a circle of closely-spaced heavy bolts to apply pressure to the gaskets against the opposing internal pressure in the header space. Even so, leaks often develop in a short time, particularly when the heat exchanger is subjected to frequent cycles of operation.
Because of the difficulty in maintaining fluid-tight sealing with gaskets, it has become common practice to weld the diaphragm into the access opening. Welding is a time-consuming and expensive procedure, and the welds must be carefully inspected. When the sealing member must be removed for inspection or repair of the tubes, the weld must be chipped and ground out, and the mating surface must be, in many instances, filled with weld metal and remachined. This time consuming procedure may lead to postponement of regular inspections, with possibly serious consequences. In addition, after a number of operating cycles, weld seals may crack as the result of repeated thermal expansions and contractions.